1. Field of the Invention
This invention relates to the field of surveillance systems, and in particular to a system that includes both RFID and Video tracking capabilities. Of particular note, this invention addresses the joint calibration of such systems and the subsequent application of this calibration in tracking objects in a surveillance area.
2. Description of Related Art
The use of video cameras for tracking objects in a surveillance area is common in the art. The “actual” location of an object (also commonly referred to as the “real-world” coordinates of the object, the “physical” location of the object, the “3-D” coordinates of the object, and so on) is determined from the appearance of the object in the field of view of one or more cameras based on a “calibration” of the one or more cameras with respect to the physical environment. “A Versatile Camera Calibration Technique for High-Accuracy 3D Machine Vision Metrology Using Off-the-Shelf TV Cameras and Lenses” by R. Y. Tsai, published in the IEEE Transactions on Robotics and Automation (3), No. 4, 1987, at pages 323–344 discloses a complete camera calibration procedure based on a set of corresponding points in a physical environment and the image of the environment in the camera's field of view.
Camera location determining systems have a number of limitations, particularly in a complex physical environment that limits the fields of view of the video cameras, or limits the particular placement of each video camera. Directions to the left, right, up, and down relative to the orientation of the camera are apparent from the camera image, but the range or distance from the camera requires a determination based on other parameters, such as the apparent size of the object in the image. Additionally, camera location determining systems require a line-of-sight view of each tracked object.
Radio frequency identification (RFID) tags are also commonly used to track objects in a surveillance area. The RFID tag typically comprises a transponder that receives RF energy from a transmitter and responds with an identification signal, using the received RF energy to provide the response. By placing multiple receivers throughout the surveillance area, radio direction-finding techniques can be employed to determine the location of each RFID tag as it traverses the surveillance area. For the purposes of this disclosure, the terms RFID and RFID tag are used herein to include any emitter of RF energy that is tracked by an RF-based surveillance system. U.S. Pat. No. 6,154,139, “METHOD AND SYSTEM FOR LOCATING SUBJECTS WITHIN A TRACKING ENVIRONMENT”, issued 28 Nov. 2000 to Alan C. Heller, and incorporated by reference herein, discloses a locating and tracking system that uses both RFID and infrared emissions to locate tracked objects.
RFID location determining systems have a number of limitations, particularly if the RFID tag is moving during the location determining process. Generally, the location determining process is based on the time of arrival of the response signal from the tag at each receiver, assuming an ideal straight-line path between the tag and each receiver. Physical structures, noise and reflection sources, and other factors, will each affect either the actual time of arrival of the response signal, or the accuracy of determining the time of arrival of the response signal at each receiver. Further, when the tag is moving, its reported location will lag its current location, and the amount of lag will be dependent upon the time required to read and process the tag information or otherwise identify the RF transmission. This reading and processing time may vary with the speed of movement of the RFID tag, with the number of other RFID tags being concurrently processed, and so on.